Soot blower lance

ABSTRACT

A tube having a central flow section for carrying a fluid and a concentric annular flow section for carrying a fluid, e.g., a lined boiler soot blower, is disclosed as being provided with means for maintaining the velocity of fluid in the annular flow section at a pre-selected substantially constant value, so as to minimize the pressure differential between the central flow section and the concentric annular flow section of the tube.

United States Patent [191 Karnofsky SOOT BLOWER LANCE [75] Inventor: George B. Karnofsky, Pittsburgh,

[73] Assignee: Capes-Vulcan, Inc., Lake City, Pa.

[22] Filed: Aug. 4, 1972 [211 Appl. No.: 278,100

[52] US. Cl 15/317, 122/392, 239/D1G. 13 [51] Int. Cl. F23j 3/00, F27d 23/00 [58] Field of Search 15/316, 317, 318;

239/DIG. 13; 122/392 [56] 1 References Cited UNITED STATES PATENTS 2,324,785 7/1943 Linaker 15/317 [111 3,816,871 June 18, 1974 12/1962 Evans 15/317 3,477,085

11/1969 Dulait 15/317 Primary Examiner-Edward L. Roberts Attorney, Agent, or Firm-Marn & Jangarathis [5 7] ABSTRACT A tube having a central flow section for carrying a fluid and a concentric annular 110w section for carrying a fluid, e.g., a lined boiler soot blower, is disclosed as being provided with means for maintaining the velocity of fluid in the annular flow section at a preselected substantially constant value, so as to minimize the pressure differential between the central flow section and the concentric annular flow section of the tube.

13 Claims, 4 Drawing Figures SOOT BLOWER LANCE BACKGROUND OF THE INVENTION This invention relates to soot blowers and more particularly to very long soot blower lances of the retractable type such as are now required for high-capacity boilers used in central power stations.

The conventional soot blower of the retractable type is comprised of a lance which periodically is thrusted into the boiler. At the tip end of the lance, on its cylindrical surface, there is provided an orifice through which steam or compressed air exits at high velocity. The lance is preferably rotated about its longitudinal axis during its motion across the boiler, so that the gas exiting from the orifice cleans soot from the boiler tubes in the neighborhood of the lance. In order to contain the compressed steam or air, the supported end of the lance is closed with a stuffing box that slides on a fixed tube of diameter smaller than the lance. Compressed gas is fed to the outer end of the fixed tube. The casing that supports the lance and the fixed tube, the means for rotating the lance and the means for supporting the cantilever load of the lance are well known in the art.

As the length of the lance is increased, the problems encountered in designing a lance are increased far greater than in proportion to the length. Since the bending moment at the point of support of a cantilever increases as a factor of its length, the resulting stress on the lance tube can be held within the allowable limit only by increasing the wall thickness and increasing the diameter of the tube. This increases the weight of the lance. Since the lance operates at the high pressure at least 300 psi gage required for the soot-blowing medium to exit through the orifice at high velocity, the lance tube must be designed to withstand hoop stress; and this too contributes to increasing wall thickness and weight as the diameter is increased. For any given allowable stress in the material of construction of the lance, there is an optimum lance diameter, which increases as the length of the lance increases.

It is well known to the art to construct the lance with constant outer diameter and varying inner diameter so that its wall is thinnest at the tip of the cantilever and thickest at its supported end. Most desirable from the design point of view is a continuous taper; as a practical matter lances are made in two or more sections, each section of uniform wall thickness, the sections decreasing in wall thickness from supported end to tip.

The other major factor in lance design is the maximum allowable temperature of the lance, beyond which the usual materials of construction lose their tensile strength. Since the lance enters into a boiler setting, where it is exposed to high-temperature radiant heat energy, it will quickly be destroyed unless cooled. The only practical cooling medium is the compressed air or steam which passes through the lance and out through the orifice. In power plant practice, steam at high pressure and at 800 F., taken directly from the boiler, may be preferred soot-blowing medium. Compressed air is also employed. The maximum allowable temperature of the wall of the lance is usually about 1,200 F. Hence it is essential that the cooling capability of the steam entering at 800 F. be used to its utmost. The amount of steam that passes through the lance is preferably not more than the amount that is required for soot-blowing.

It is not economically desirable to use more steam merely for the purpose of cooling the lance.

The problem becomes particularly acute as the lance length, and consequently its diameter, is increased because the surface receiving the radiant heat increases in direct proportion to the diameter and length. Also, as the diameter is increased and the amount of steam or air flowing through the lance is not increased, the heat transfer coefficient from the outer wall of the lance to the gas decreases as a consequence of reduced velocity.

The most practical means for increasing the velocity of soot-blowing medium flowing past the inner wall of the lance is to provide an inner tube which creates an annular space through which the: steam flows. Steam exiting from the fixed tube, previously described, enters into the inner tube, which is spaced from the lance, sealed to it at the tip end and opened at the supported end. The medium is thus constrained to flow through said inner tube to the support end and then through an annular space the whole length of the lance to the orifice. Thus, in any position of the lance, and particularly in the extended position when it is receiving the most heat, there is optimum transfer of heat from the wall of the tube to the medium. Such a lance is described in US. Pat. No. 3,068,507 to R. H. Evans.

Although the concept of an inner tube is not new to the art, the basis for its selection has not been well understood. In order to achieve the desired heat transfer coefficient to the medium flowing within the lance, particularly for very long lances, the velocity of the medium that is required is so high that there is substantial pressure drop from the supported end to the tip end of the annular space. Since the pressure inside the inner tube along its whole length may be approximately the same as that at the supported end of the annular space, there may be considerable pressure difference, as much as psi, between the inside and the outside of the inner tube at the tip end. In effect the inner tube becomes a pressure vessel of notable wall thickness, whose weight is a substantial portion of the total weight of the lance.

Accordingly, the problem of developing a retractable soot blower for use in the new larger boilers can been seen to be a multifaceted one wherein apparent solutions to some of the facets cause intensification of the seriousness of the others. Further, the problem presented by the need for the development of improved boiler soot blowers is illustrative of the problems faced by those in many arts wherein concentric tubing of fluids is utilized.

OBJECTS OF THE INVENTION It is the general object of the present invention, therefore, to overcome the above-described problems both with respect to tubing generally and, soot blower lances in particular.

Another object of the present invention is to provide a means for increasing the steam velocity without adding weight to the lance.

SUMMARY OF THE INVENTION The general object and others which will be obvious to those having ordinary skill in the art are achieved by providing a fluid carrying lined tube having a central flow section and an annular flow section, wherein the velocity of the fluid in the annular flow section is maintained at a substantially constant level and the pressure differential between the central flow section and the annular flow section is maintained at a minimum by the provision of selectively located, sized orifices in the liner. The orifices allow for the controlled passage of cleaning medium from the central flow section to the annular flow section of the tube, which passage selectively increases the amount of the fluid in the annular flow section thereby regulating both the velocity of the fluid therein and the pressure differential between the central and annular flow sections.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of the present invention and its many advantages will become clear from a consideration of the following detailed description thereof when read in the light of the accompanying drawings wherein:

FIG. I is a partial cross-sectional, schematic view of a boiler having a soot blower with a lance structured in accordance with the teaching of the present invention, the soot blower being shown both in partially withdrawn (solid lines) and fully inserted (dash lines) positions;

FIG. 2 is a cross-sectional elevational view of one embodiment of a soot blower structured in accordance with the teaching of the present invention;

FIG. 3 is a cross-sectional view through the plane 3-3 of FIG. 2; and

FIG. 4 is a cross-sectional elevational view of a second embodiment of a soot blower having a lance structured on accordance with the teaching of the present invention.

DETAILED DESCRIPTION Referring to FIG. 1, there is shown a soot blower, generally designated by the reference numeral comprising a casing H2 and a retractable lance generally designated by the reference numeral 114, structured in accordance with the teaching of the present invention. The soot blower I0 is shown positioned adjacent one wall 16 of a boiler with the lance 14 extending through wall 16 toward opposed boiler wall 17. The position of lance M, as shown in solid lines, corresponds to the position of the lance when substantially fully withdrawn from the boiler and the position of lance 114, as shown in dashed lines, corresponds to the position of the lance when substantially fully inserted into the boiler toward wall 17.

As is schematically depicted in FIG. ll, lance 14 when extended to the position shown in dashed lines defines a cantilever of length L which corresponds to the distance between lance support apparatus 22 and the head end 23 of the lance. Also schematically depicted in FIG. 1 is the fact that lance 14, when extended into the boiler to define the cantilever of length L, experiences a downward deflection D which is caused by the weight of the lance generating a bending moment at the support 22.

The casing I2 contains a traveling carriage 25 for supporting the lance 114 at end 26, a cleaning medium supply line 27 which extends from a source of cleaning medium (not shown) into lance 14 as will be discussed in detail below, the lance support apparatus 22 which defines the point at which lance 14 is cantilevered when inserted into the boiler, and drive apparatus (not shown) for causing the traveling carriage to impart reciprocating, rotational motion to lance 14 so as to accomplish the desired boiler cleaning function. Each of these elements may be any of the present invention.

Referring now to FIG. 2, one embodiment of a soot blower lance l4 structured in accordance with the present invention is shown with intermediate sections removed for the sake of clarity. The lance 14 comprises a generally cylindrical outer casing 30 which is closed at head end 23 by a dome cap 32 which is suitably secured to casing 30, such as by welding at 33. The tail end 26 of the outer casing 30 is closed by a connecting collar, generally designated by the reference numeral 36, suitably secured thereto as by bolts 37. Connecting collar 36 includes a cylindrical sleeve 39 having a radially extending flange 40 formed thereon, which flange corresponds in diameter to the outer diameter of a mating flange 41 formed on the tail end 26 of outer casing 30. The flanges 40 and 41 are provided with aligned bolt holes for accommodating the securing bolts 37. The other end of sleeve 39 is provided with a stuffing box 42 thereon for slidably receiving, in pressure tight relation, cleaning medium supply line 27 therethrough. The outer surface of stuffing box 42 is provided with an annular channel 44 for rigidly securing traveling carriage 25 thereto so as to facilitate the transmission of reciprocating and rotary motion to lance 14 as required for the cleaning operation.

Mounted concentrically within outer casing 30 is a generally cylindrical lance liner '46. Liner 46 is rigidly secured as by welding, to the inner surface of end cap 32 at the head end 23 of lance l4, and is supported within outer casing 30 by spaced, radially extending supports 47. The supports 47 are rigidly secured to the outer surface of liner 46, are in sliding contact with the inner surface of outer casing 30. The supports 47 are provided in sets of three (FIG. 3) to extend radially outwardly from liner 46 at intervals therearound. The longitudinal axial spacing of the liner supports 47 is determined based upon the structure of each individual liner, i.e., a sufficient number of supports are pro vided so that the liner 46 is uniformly stressed and not allowed to buckle or sag. Such buckling or sagging is undesirable, since it would quickly bring about the destruction of the liner 46 because a non-uniform area of the annular space between casing as and liner 46 would cause localized hot spots on the outer casing 30.

Rigidly secured to the inner surface of liner 46 are a plurality of longitudinally axially spaced bearing elements 50 (FIGS. 2 and 3) for guiding and supporting the cleaning medium supply line 27 within lance liner 46. Each bearing element 50 comprises an insert in the form of a hubbed wheel having an outer ring 51 and a concentrically inner ring or hub 52 connected by radially extending fins 53. The inner rings or hubs 52 of bearing elements 50 are tapered and serve as the hearing surfaces for cleaning medium supply line 27 which is also tapered at the outlet end to permit easy access of the supply tube 27 into each bearing element as the lance is retracted despite relative bending of the lance 46 and the supply line 27. The radially extending fins 53, which are radially aligned with the supports 47 carry the unsupported weight of line 27 to the lance liner 46 and thence through supports 47, to the outer casing 30. As was the case with supports 47, the fins 53 of bearing elements 50 are provided in sets of three and extend radially between the inner ring or hub 52 and the outer ring 51 at 120 intervals. It is desirable to have as few of the supports 47 and bearing elements 50 as possible in fact, although small, each set of bearing elements adds to the weight of the lance which must be supported as the above-described cantilever beam.

Upon determining the locations of the bearing elements 5!), the bearing elements are secured to the liner 46, such as by spot-welding, and suitable holes 62 are provided about the lance liner 46.

As is clearly shown in FIG. 2, the outer casing 30 of lance T4 is thinnest at the head end 23 of the lance and thickest at the tail end 26 thereof which is the supported end. The thinning of the wall thickness of the outer casing in this embodiment is accomplished by providing a plurality of steps 54 and 55 at spaced intervals along the axial length of the outer casing 30. Steps S4 and 55 may be formed by manufacturing outer casing 30 in sections, the sections being joined in any suitable manner, such as by a welded butt and socket joint, or by any other manufacturing process suitable for providing the desired steps in the casing without sacrificing structural integrity.

Formed adjacent the head end 23 of outer casing 30 are two diametrically opposed nozzles 60 for directing the cleaning medium against the boiler tubes to be cleaned. The nozzles 60, as shown, are directed radially outwardly normal to the longitudinal axis of the lance 14. It is to be recognized, however, that virtually any nozzle arrangement may be utilized without departing from the scope of the present invention. Further, it is also to be noted that the nozzles 60 and formed integrally with outer casing 30, however, in most cases, they may comprise inserts which are replaceable and which may be designed for any particular cleaning application as required.

As seen in FIG. 2, a plurality of orifices 62 and 64 are provided in liner 46 substantially adjacent each of the steps 55 and 54, respectively, in outer casing 30. As will be discussed in detail, these orifices provide for the passage of cleaning medium from a central flow section 45 within liner 46 to an annular flow section 48 defined by the space between liner 46 and outer casing 30 in ac cordance with the present invention.

In the operation of the soot blower lance 14 of FIGS. 2 and 3, a cleaning medium, which may be steam or compressed air, is introduced through cleaning medium supply line 27 into the central flow section 45. A major portion of the cleaning medium thereafter passes back around supply line 27 within liner 46 toward the upstream end of the lance T4, i.e., toward the right as seen in FIGS. l and 2. Thereafter, such major portion of the cleaning medium continues upstream and out the open end of liner 46 into outer casing 30 and begins to travel downstream within the annular flow section 48 for the full length of the lance 14. When the cleaning medium has traveled the full length of the lance 14, it is discharged through nozzles 60 against the boiler tubes to accomplish the desired cleaning function.

It is evident from FIG. 2 that the increases in inside diameter of outer casing 36 which occur at steps 54 and 55 cause increases in the volume per unit length of annular flow section 48. Clearly such volume increases cause, in the absence of corrective measures, a reduction in the velocity of the cleaning medium flowing through the annular flow section 48, which reduction is undesirable for the reasons discussed in detail above. Further, in that the length L (FIG. 1) of soot blower lances for high capacity boilers is contemplated to be as much as 50 feet and more, the pressure drop of the cleaning medium as it passes downstream within the annular flow section 48 of the lance 14 will be significant. Such a pressure drop generates a pressure differential between the central flow section 45 and annular flow section 48 of the lance 14 which generates significant hoop stresses in the liner 46, particularly adjacent the head end 32 of the lance 14 where the pressure of the cleaning medium in the annular flow section 48 is least. Thus, in the absence of appropriate corrective measures, the liner 46 would have to be structured such as to be capable of supporting the hoop stresses which, as noted above, would generate other problems. The present invention, however, overcomes these problems by providing the orifices 62 and 64 in the lance liner 46.

More particularly, the provision of orifices 62 and 64 in the liner 46 allows the cleaning medium to pass in a controlled manner from within the liner 46 to the annular flow section 48 in the area of steps 54 and 55. The passage of the cleaning medium into the annular flow section 48 at the orifices 62 and 64 adds a quantity of cleaning medium to that already flowing in the annular flow section 48 in amounts which substantially correspond to the volumetric increase in the size of the annular flow section 48 to maintain the velocity of the cleaning medium through the annular flow section 48 substantially constant. Thus, each change in unit volume of the annular flow section 48 at the respective steps 54 and 55, which would otherwise cause a reduction in the velocity of the cleaning medium, is compensated for by the addition of an appropriate amount of cleaning medium to that flowing within the annular flow section 48, which additional cleaning medium causes the velocity of the cleaning medium passing through the annular flow section 48 to remain substantially constant. Accordingly, the optimum amount of the cleaning medium for boiler cleaning for any given application is determined by experience and the desired velocity in the annular flow section 48 that is required for heat transfer from lance to cleaning medium as calculated by well known methods. Thereafter, by utilizing the technique of the present invention, the predetermined optimum velocity can be maintained notwithstanding variations in the linear volume of the annular flow chamber of the lance 14.

In addition to maintaining the flow velocity of the cleaning medium constant, the introduction of additional cleaning medium into the annular flow section 48 through orifices 62 and 64 also maintains the pressure of the cleaning medium in the downstream portion of the annular flow section 48 as near as possible to the pressure in the liner 46 at any given position. Such a reduction in pressure differential clearly reduces the hoop stress to which the liner 46 is subjected and enables a reduction in liner wall thickness to an absolute minimum. Obviously, such a reduction in liner wall thickness reduces the liner weight and therewith the cantilever load on outer casing 30.

Referring now to FIG. 4 there is shown a second embodiment of a soot blower lance structured in accordance with the teaching of the present invention and generally designated by the reference numeral 114. The lance 114 is, structurally substantially the same as lance 14 of FIG. 2 and comprises generally an outer casing 130, a lance liner 146 mounted coaxially concentrically within outer casing 130 and cooperating therewith to define an annular flow section 148, and a cleaning medium supply line 127 extending coaxially concentrically within liner 1% into a central flow section 1145 defined thereby. Similarly to liner 46 of lance l4, liner 1% is positioned within casing 130 by a plurality of support elements 147 suitably positioned therebetween. Additionally, the inner surface of lance 146 is provided with a plurality of axially spaced bearing elements 150 for guiding and supporting cleaning medium supply line 127 when it is positioned within the lance 114. Outer casing 130 adjacent its head end 123 is provided with a pair of nozzles 160 for directing a cleaning medium against the boiler tubes during the operation of the soot blower.

The basic difference between the embodiment of FIG. 4 and the embodiment of FIG. 2, is that in FIG. 4 the inner surface of outer casing 130 comprises a continuously tapered tube whereas the inner surface of outer casing 39 of lance 14 (FIG. 2) is stepped. It can be seen that in tapered casing 130, the linear volume of the annular flow section 148 of lance H4 changes continuously from the tail end H285 to the head end 123 of the lance M4. It is clear, therefore, that in the embodiment of FIG. 4, the tendency of the velocity of cleaning medium flowing in the annular flow section 148 is a gradual change rather than a substantially sudden change as is experienced with the stepped volume changes of the embodiment of FIG. 2. Thus, the orifice 163 in liner 1% are suitably located to provide a gradual introduction of cleaning medium from within the central flow section M5 to the annular flow section I48 along the length of liner M6. In all other respects, however, the structure and operation of the embodiments of FIGS. 2 and 4 are identical.

It is to be noted that the size of the orifices 62 and 64, and 163 of the lance embodiments of FIGS. 2 and 41, respectively, are shown as being quite small. It is preferable to drill a plurality of small holes in the liner rather than a lesser number of larger holes for two reasons: first, large holes tend to weaken the tube more than a plurality of small holes; and secondly, since the radial dimension of the annular flow sections 48 and M8 is not very wide, e.g., in the order of 0.2 inches, the utilization of small holes allows sizing and therewith control to be determined by the well known orifice flow equation, which equation would not apply in the absence of space for a vena contracta to form outside the outer surface of liners 46 and 146. Thus, small holes provide the advantage of increased strength and greater predictability of operation which would not otherwise be available with larger holes.

It can be seen from the foregoing, therefore, that the teaching of the present invention enables the construction of longer retractable soot blower lances while at the same time maintaining the capability of the lance material to both withstand the environmental heat experiences within the boiler and to support the stresses generated by the cantilever lance when it is fully extended. The velocity of cleaning medium necessary to achieve optimum flow based upon both cleaning and cooling consideration can be determined, established and maintained so as to provide soot blower capability at relatively low initial expenditure and with relatively low operating costs.

What is claimed is:

I. In a lance for a boiler soot blower, said lance comprising:

an outer casing including flow nozzles mounted about a head end thereof and a liner mounted therein, said outer casing including closure means about said head end, said liner including closure means at an end thereof proximate to the head end of said outer casing, said liner defining a central flow section within said lance, said liner and said outer casing cooperating to define an annular flow section within said lance, and cleaning medium supply means extending into said central flow section for introducing a cleaning medium to said lance, the improvement comprising:

means formed in said liner for placing said central flow section in fluid communication with said annular flow section to maintain the velocity of said cleaning medium through said annular flow section substantially constant; and

means for enabling said cleaning medium to pass through said flow nozzles.

2. The soot blower lance as claimed in claim 1 wherein said annular flow section varies in crosssectional area along the length thereof.

3. The soot blower lance as claimed in claim 2 wherein said variation in cross-sectional area of said annular flow section is defined by variations of the wall thickness of said outer casing, and said means for maintaining said velocity constant are positioned in relation to said variation in said wall thickness.

4. The soot blower lance as claimed in claim 2 wherein said variations in the cross-sectional area of said annular flow section are defined by steps formed on the internal surface of said outer casing, and said means for maintaining said velocity constant are positioned substantially adjacent said steps.

5. The improvement as claimed in claim 1 wherein said means for maintaining said velocity constant comprise orifices formed in said liner.

6. The improvement as claimed in claim 5 wherein said orifices are sized such as to develop vena contracta in cleaning medium flowing therethrough.

7. In a lance for a boiler soot blower, said lance comprising:

an outer casing including flow nozzles mounted about a head end thereof and a liner mounted therein, said outer casing including closure means about said head end, said liner including closure means at an end thereof proximate to the head end of said outer casing, said liner defining a central flow section within said lance, said liner and said outer casing cooperating to define an annular flow section within said lance, and cleaning medium supply means extending into said central flow section for introducing a cleaning medium to said lance, the improvement comprising:

means formed in said liner for placing said central flow section in fluid communication with said annular flow section to minimize the build-up of a pressure differential between said cleaning medium in said central flow section and said cleaning medium in said annular flow section; and

means for enabling said cleaning medium to pass through said flow nozzles.

8. The improvement as claimed in claim 7 wherein said means for precluding the build-up of a pressure differential are also for maintaining the velocity of said cleaning medium through said annular flow section substantially constant.

9. The improvement as claimed in claim 7 wherein said means for precluding the build-up of a pressure differential are orifices formed in said liner.

10. The improvement as claimed in claim 8 wherein said means for precluding the build-up of a pressure differential comprise a plurality of spaced orifices.

11. A lance for retractable soot blower comprising:

a cylindrical outer casing, said cylindrical outer casing having a head end and a tail end, said cylindrical outer casing becoming progressively thinner from said tail end to said head end, said cylindrical outer casing being enclosed about said head end and provided with flow nozzle means proximate thereto;

a cylindrical liner mounted cylindrically within said cylindrical outer casing, said cylindrical liner defining a central flow section within said lance and being enclosed about an end proximate to the head end of said casing;

said cylindrical liner and said cylindrical outer casing cooperating to define an annular flow section within said lance, said annular flow section increas ing in cross-sectional area from. said tail end to said head end of said cylindrical outer casing;

cleaning medium supply means extending coaxially within said liner, said supply means for introducing a cleaning medium to said lance;

means for positioning said cylindrical liner coaxially within said cylindrical outer casing;

means for positioning said cleaning medium supply line within said liner;

means formed in said cylindrical liner for communicating said central flow section with said annular flow section; and

means for enabling said cleaning means to pass through said flow nozzles.

12. The lance as claimed in claim 11 wherein said means for communicating said central flow section and said annular flow section comprises orifices formed in said liner.

13. The lance as claimed in claim 12 wherein said orifices are disposed along said liner in relation to the variation in the cross-sectional area of said annular flow section. 

1. In a lance for a boiler soot blower, said lance comprising: an outer casing including flow nozzles mounted about a head end thereof and a liner mounted therein, said outer casing including closure means about said head end, said liner including closure means at an end thereof proximate to the head end of said outer casing, said liner defining a central flow section within said lance, said liner and said outer casing cooperating to define an annular flow section within said lance, and cleaning medium supply means extending into said central flow section for introducing a cleaning medium to said lance, the improvement comprising: means formed in said liner for placing said central flow section in fluid communication with said annular flow section to maintain the velocity of said cleaning medium through said annular flow section substantially constant; and means for enabling said cleaning medium to pass through said flow nozzles.
 2. The soot blower lance as claimed in claim 1 wherein said annular flow section varies in cross-sectional area along the length thereof.
 3. The soot blower lance as claimed in claim 2 wherein said variation in cross-sectional area of said annular flow section is defined by variations of the wall thickness of said outer casing, and said means for maintaining said velocity constant are positioned in relation to said variation in said wall thickness.
 4. The soot blower lance as claimed in claim 2 wherein said variations in the cross-sectional area of said annular flow section are defined by steps formed on the internal surface of said outer casing, and said means for maintaining said velocity constant are positioned substantially adjacent said steps.
 5. The improvement as claimed in claim 1 wherein said means for maintaining said velocity constant comprise orifices formed in said liner.
 6. The improvement as claimed in claim 5 wherein said orifices are sized such as to develop vena contracta in cleaning medium flowing therethrough.
 7. In a lance for a boiler soot blower, said lance comprising: an outer casing including flow nozzles mounted about a head end thereof and a liner mounted therein, said outer casing including closure means about said head end, said liner including closure means at an end thereof proximate to the head end of said outer casing, said liner defining a central flow section within said lance, said liner and said outer casing cooperating to define an annular flow section within said lance, and cleaning medium supply means extending into said central flow section for introducing a cleaning medium to said lance, the improvement comprising: means formed in said liner for placing said central flow section in fluid communication with said annular flow section to minimize the build-up of a pressure differential between said cleaning medium in said central flow section and said cleaning medium in said annular flow section; aNd means for enabling said cleaning medium to pass through said flow nozzles.
 8. The improvement as claimed in claim 7 wherein said means for precluding the build-up of a pressure differential are also for maintaining the velocity of said cleaning medium through said annular flow section substantially constant.
 9. The improvement as claimed in claim 7 wherein said means for precluding the build-up of a pressure differential are orifices formed in said liner.
 10. The improvement as claimed in claim 8 wherein said means for precluding the build-up of a pressure differential comprise a plurality of spaced orifices.
 11. A lance for retractable soot blower comprising: a cylindrical outer casing, said cylindrical outer casing having a head end and a tail end, said cylindrical outer casing becoming progressively thinner from said tail end to said head end, said cylindrical outer casing being enclosed about said head end and provided with flow nozzle means proximate thereto; a cylindrical liner mounted cylindrically within said cylindrical outer casing, said cylindrical liner defining a central flow section within said lance and being enclosed about an end proximate to the head end of said casing; said cylindrical liner and said cylindrical outer casing cooperating to define an annular flow section within said lance, said annular flow section increasing in cross-sectional area from said tail end to said head end of said cylindrical outer casing; cleaning medium supply means extending coaxially within said liner, said supply means for introducing a cleaning medium to said lance; means for positioning said cylindrical liner coaxially within said cylindrical outer casing; means for positioning said cleaning medium supply line within said liner; means formed in said cylindrical liner for communicating said central flow section with said annular flow section; and means for enabling said cleaning means to pass through said flow nozzles.
 12. The lance as claimed in claim 11 wherein said means for communicating said central flow section and said annular flow section comprises orifices formed in said liner.
 13. The lance as claimed in claim 12 wherein said orifices are disposed along said liner in relation to the variation in the cross-sectional area of said annular flow section. 